This invention relates to electric fuses, and more particularly to exothermically assisted electric fuses wherein the heat energy released during exothermically assisted operation is directed toward the fusible element.
Current limiting power interruption in high voltage circuits requires a current interruption device which rapidly brings the current to a zero value upon the occurrence of a line fault. The fuse devices generally considered herein are those employed in electrical circuits typically exhibiting voltages in excess of 1,000 volts. It should be apparent that, for electrical circuits operating at such high energy levels, extensive damage can occur to numerous circuit components and machinery connected to the circuit, or to various other portions of an electrical energy distribution system, if current interruption is not accomplished in a short period of time following the occurrence of various line conditions which draw high current. It is generally desirable to reliably bring such currents to a zero value within the shortest possible time upon the occurrence of an event such as a short circuit line fault.
Conventional high voltage current-limiting fuses generally consist of an electrically conductive fusible element, such as a silver ribbon or wire, wound on a supporting electrically insulative structure. When an overcurrent of a predetermined level flows through the fusible element for a predetermined duration, the fusible element melts at one or more restricted locations along its length, establishing an arc in each region where melting occurs. In this manner, a multiplicity of series-connected arcs is formed in the fuse. Current interruption in this type of fuse occurs when the sum of the individual arc voltages exceeds the voltage applied to the fuse. However, if the overcurrent is relatively small compared to the continuous current rating of the fuse, such as, for example, an overcurrent which is 1.5 times the continuous current rating, the fusible element may melt at only one location, so that only a single arc is created in response to the overcurrent condition. For the fuse to successfully interrupt a high voltage current using a single arc, the arc length must be increased to a relatively long length in a short period of time. For example, to interrupt 15 kV using a single arc, the arc length must be in the range of 25.4 to 76.2 centimeters (10 to 30 inches). Developing such a long arc within the required time is not usually feasible, considering the slowness with which the arc will elongate when the current density is relatively low. Accordingly, it is highly desirable to reliably and simultaneously open the fusible element at a number of positions along its length, so as to create the number of arcs required for interruption.
Various means have been used in the past to establish multiple breaks in a high voltage fusible element in order to facilitate low overcurrent interruption by the fuse. One such means is described in U.S. Pat. No. 4,357,588 to J. G. Leach et al., which is assigned to the same assignee as the present application. In this patent, fusible elements are disclosed which include portions having reduced cross-sectional area. The reduced cross-sectional area portions have a desired fusible time-current characteristic for causing rupturing of the fusible elements, so that the fusible elements disclosed are especially suitable for low overcurrent fault interruption. However, even though the reduced cross-sectional area portions of the disclosed fusible elements make them especially suitable for low current fault interruption, there is still a minimum current density required in the reduced cross-sectional area portions for melting to occur in more than one location. This minimum current density corresponds to a melting time of between 1 and 2 hours. As noted hereinabove, it is desirable that a high voltage fuse be capable of interrupting any current which causes the fusible element to open.
Another approach for achieving multiple breaks in the fusible element in response to persistent overcurrents of low value is to exothermically assist the operation of the fuse, by employing an exothermic material to melt or blast away the fusible element at selected locations along its length upon disruption of the fusible element by an overcurrent condition. For example, exothermically assisted electric fuses are described in U.S. Pat. No. 3,705,373 to F. L. Cameron, U.S. Pat. No. 4,176,385 to R. Dethlefsen, U.S. Pat. No. 3,958,206 to R. V. Klint, and U.S. Pat. No. 4,486,734 to J. G. Leach. The latter two patents are assigned to the same assignee as the present application. Such fuses typically include one or more exothermic bodies disposed in heat transfer relationship with at least a portion of the fusible element of the fuse, and a triggering circuit for initiating an exothermic reaction in the exothermic bodies in response to an overcurrent through the fusible element. The present invention is directed to improving this technique of providing multiple openings in the fusible element, by properly directing the energy released by the exothermic material toward the fusible element. Doing so reduces the amount of exothermic material needed. The present invention is also directed to protecting the exothermic material from the mechanical stresses of fuse assembly.
Accordingly, it is an object of the present invention to provide an exothermically assisted electric fuse in which the heat energy released during exothermically assisted operation is directed toward the fusible element.
Another object of the present invention is to provide an exothermically assisted electric fuse with reduced mechanical stress.
A further object of the present invention to provide an exothermically assisted electric fuse requiring a reduced amount of exothermic material.